The present invention relates generally to measuring the voltage level of, and characterizing, an unknown voltage source. More particularly, the present invention relates to measuring the voltage of an unknown voltage source, particularly a battery, in an electronic device. This enables characterization as to battery life remaining, or in the cases of other kinds of power supplies, monitoring them and characterizing them as to whether they are within operational parameters.
Many electronic devices use a portable DC power supply, most of which use battery power. Such electronic devices can use chemical batteries, including rechargeable batteries, or other known types. In many such devices, it is important to be able to measure the potential or instantaneous voltage, i.e. the xe2x80x9clevelxe2x80x9d of the battery. Being able to measure the battery level enables a user to have a rough prediction of remaining battery life. As will be appreciated, this is because the potential decreases in a known way as a battery of a particular type slowly discharges in normal use.
Knowing the battery level is especially important in certain devices because data can be lost if the device shuts down unexpectedly. For example, in a laptop computer application, a user typically desires to know the battery life remaining at all times. When the battery voltage drops below a certain level, the volatile memory in the laptop computer will be erased, and/or the laptop will shut off. This is also true with respect to wireless instruments such as cellular telephones. It is desirable for the user to know the approximate length of the remaining battery life because it is frustrating to have a conversation terminated abruptly due to an expended battery. If users of such electronic devices have an accurate measurement of battery level, and thereby a characterization of the battery such as a prediction of the battery life remaining, they are less likely to be subjected to data loss or communication interruptions, assuming rational user behavior.
There are a number of voltage level measuring devices known in the relevant art. These devices typically can have one or both of two drawbacks; namely, that they tend to have a low level of accuracy, or that they are expensive to build. In the competitive electronics marketplace, it is likely quite valuable to provide a low-cost, accurate voltage measuring device. However, in the past the design goals of accuracy and low cost have appeared to be mutually exclusive. For example, known voltage measuring devices having increased accuracy generally are more complex compared with less accurate devices; and typically require several relatively expensive components. These factors increase the cost. Examples of devices which measure unknown voltage levels from a power source are disclosed in U.S. Pat. No. 4,071,822 to Kamiya, and U.S. Pat. No. 3,999,123 to Thoener.
It has been recognized that it would be advantageous to provide a device which uses inexpensive electronic components and a straight-forward method to characterize an unknown voltage source, e.g. to determine battery voltage in an electronic device. In a more detailed aspect, it is also desirable to provide a system which would allow accurate measurement of an unknown voltage source without disconnecting a test capacitor from the circuit. In a further more detailed aspect, it has been recognized that it would be advantageous to develop a system which minimizes the use of relatively expensive electronic components, such as A/D converters to measure the voltage output of an unknown voltage source such as a battery.
In accordance with one aspect of the present invention, the system includes a voltage measuring circuit comprising a capacitor configured for storing a charge from the unknown voltage source. The capacitor is charged through a resistor. A switch, which can be a solid-state device such as a transistor, is configured in parallel to discharge the capacitor at a time after the capacitor has reached a threshold voltage. A microprocessor controls the switch and measures the amount of time required to charge the capacitor to the threshold voltage. The time required to charge the capacitor to the threshold potential is used to determine the voltage level of the unknown voltage source. In accordance with another aspect of the present invention, the system can include a voltage comparator coupled to the microprocessor to signal the instant in time when the threshold voltage is reached.
Another aspect of this invention is a method for measuring the voltage level of an unknown voltage source. One step of the method is charging a previously discharged capacitor, beginning at a known instant of time, from the unknown voltage source. Another step of the method is monitoring the amount of time needed to charge the capacitor from a discharged state to a threshold voltage level without disconnecting the capacitor from the unknown voltage source. In another step, the capacitor voltage is discharged using a switch electrically coupled in parallel after the capacitor voltage has reached the threshold value. The method also includes the step of determining the unknown voltage based on the length of time required to charge the capacitor from a fully discharged state to the threshold voltage level.
Additional features and advantages of the invention will be appreciated with reference to the following detailed description, which, taken in conjunction with the accompanying drawings, illustrate, by way of example, features and advantages of the invention.